The Python fatal errors occur only in a debug build now.

In a release build, unghostify() raises a Python SystemError
exception if it detects insanity, and ghostify() ignores the
problem (as ZODB 3.2 did, although it looks like 3.2 ignored
the problem by mistake).  This isn't *good*, because the
system is insane.  But it can become insane only by breaking
absolute threading rules, and that "should never happen".
Unfortunately, hundreds of things can go wrong if it does
happen, and we can't detect most of them.